When I've chosen to unfreeze, I simply logged on to the account using my ID and (strong) password and got options to choose from on the screen. A couple of confirmations and I can unfreeze either for a specific entity or a specific date range.
There are provisions in case you forget your ID or password (those magical "Mother's Maiden Name" or "High School Mascot" questions). In fact, I just had to recreate my account on TransUnion because, apparently, I haven't logged in in a year (time flies when you're having fun...). As always, make the answers to the questions bogus, like making "High School Mascot" not "Lions" or whatever it was, but "critters" or even the nonsense "bleen". Just write those down somewhere safe so you have access to the answer(s), because they have to match exactly.